1. Field of the Invention
The present invention relates to phase shifting masks and, in particular, to the use of model-based tools to facilitate phase assignment on the phase shifting masks.
2. Discussion of the Related Art
Lithography is a well-known process used in the semiconductor industry to form lines, contacts, and other known structures in integrated circuits (ICs). In conventional lithography, a mask (wherein the term “mask” as used herein can refer to a mask or a reticle) having a pattern of transparent and opaque regions representing such structures in one IC layer is illuminated. The emanating light from the mask is then focused onto a photoresist layer provided on a wafer. During a subsequent development process, portions of the photoresist layer are removed, wherein the portions are defined by the pattern. In this manner, the pattern of the mask is transferred to (i.e. printed on) the photoresist layer.
However, diffraction effects at the transition of the transparent regions to the opaque regions on the mask can render the corresponding printed edges on the wafer indistinct, thereby adversely affecting the resolution of the lithography process. Various techniques have been proposed to improve the resolution. One such technique, phase shifting, uses phase destructive interference of the waves of incident light. Specifically, phase shifting shifts the phase of a first region of incident light waves approximately 180 degrees relative to a second, adjacent region of incident light waves to create a feature between the first and second regions. Thus, a feature, as defined by exposed and unexposed portions of a photoresist illuminated through a mask, can be more closely defined by using phase shifting, thereby allowing greater structure density on the IC. Typically, features generated by phase shifting can then be protected from exposure by using a “trim” mask, which is used to expose the remaining field.
As the need for feature density increases, phase shifting is being applied to many features on the layout. In one embodiment, called a full phase approach, substantially all features of a layer can be defined using phase shifting. However, using phase shifting in dense layouts can result in phase conflicts. Phase conflicts can negate the optical interference necessary to create the desired feature(s). Therefore, assigning phase to the layout can constitute a time-intensive, but mandatory part of typical process flows in the production of many integrated circuits.
Other modifications can be made to the layout to optimize printing resolution. For example, a process called optical proximity correction (OPC) can be used to compensate for non-linear distortions caused by optical diffraction and resist process effects when transferring the pattern from the mask to the wafer. Advanced computer-implemented tools can simulate (i.e. predict) a real pattern transfer with a set of mathematical formulas (i.e. models). In simulating pattern transfer, a simulation tool can use one or multiple models with the layout to generate an OPC-modified layout.
FIG. 1 illustrates a standard process flow 100 including phase assignment as well as OPC. In step 101 of process flow 100, a target layout can be provided to a phase shifting tool for analysis. In one embodiment, the iN-Phase™ tool, licensed by Numerical Technologies, Inc., can be used. The PSM type, e.g. double exposure alternating aperture phase shifting, can be designated in step 102. Step 103, in which phase shifting is applied to the target layout, can include the steps of placing the shifters in the layout (step 104), assigning phase to the shifters (step 105), and resolving any phase conflicts (step 106). Step 105 may include algorithms for assigning phases to the shifters.
The algorithms used are based phase dependencies and, optionally, costs. By describing the dependencies in one or more data structures, the algorithms can attempt to solve the problem. See, e.g., U.S. patent application Ser. No. 09/823,380, filed Mar. 29, 2001, entitled, “Incrementally Resolved Phase-shift Conflicts In Layouts For Phase-Shifted Features”, and U.S. patent application Ser. No. 10/085,759, filed Feb. 28, 2002, entitled “Design And Layout Of Phase Shifting Photolithographic Masks”, both of which are incorporated by reference herein. Performing these algorithms to assign phase can be extremely time consuming and require a separate computer program and/or software to be maintained for performing phase assignment, as opposed to the simulation engine for model-based optical proximity correction.
After being output in step 107, the phase shifted layout can then be input to a tool that applies OPC. In one embodiment, the tool for applying phase shifting is different than the tool for providing OPC. In another embodiment, the same tool can provide both phase shifting and OPC. For example, the iN-Phase™ tool, licensed by Numerical Technologies, Inc., can assign phase to shifters as well as perform OPC of a finished layout. In step 109, the layout can be modified as appropriate. At this point, the final layout can be output in step 110. Unfortunately, in either embodiment, significant time is spent reading (or streaming out) and then streaming in the phase shifted layout (steps 107 and 108).
Therefore, a need arises for an accurate and time effective method and system for assigning phase to shifters in a model-based environment.